Power semiconductor devices, in particular field-effect controlled switching devices such as a Metal Oxide Semiconductor Field Effect Transistor (MOSFET) or an Insulated Gate Bipolar Transistor (IGBT), have been used for various applications including but not limited to use as switches in power supplies and power converters, electric cars, air-conditioners, and even stereo systems. Particularly with regard to power devices capable of switching large currents and/or operating at higher voltages, low on-state resistance Ron, high breakdown voltages Ubd, and/or high robustness are often desired. A power MOSFET typically includes a drain region, a drift region adjoining the drain region, and a source region, each having a first conductivity type, and a body region arranged between the drift region and source region of a second conductivity type. A power IGBT has a similar construction as a power MOSFET, except that the first conductivity type drain region is replaced with a second conductivity type collector region, thus forming a bipolar junction transistor with a voltage controlled switch supplying the base current of the BJT.
One issue of particular concern in power switching applications is cosmic ray radiation. Cosmic ray radiation refers to unwanted particle bombardment from the exterior environment into the operational regions of the device. Although it is more prevalent in space environments, cosmic ray radiation can occur in terrestrial environments. The particle bombardments caused by cosmic ray radiation can set off a chain reaction of impact ionization, which causes unwanted current filamentation and can lead to irreversible device failure. Devices that operate with high electric field gradients, such as power switching devices, are most vulnerable to failure from cosmic ray radiation. For this reason, many power semiconductor switching applications require the device to be ruggedized against cosmic ray radiation. Mitigating high electric fields at critical locations within the power device enables robust device performance against harsh operation conditions such as cosmic ray radiation.
Techniques used to tailor the electric field profile and peak intensity of power switching devices to improve cosmic ray robustness include (i) increasing the wafer/drift region thickness; (ii) introducing a thicker graded/diffused base material profile; (iii) reducing the n-type drift region/ intrinsic layer doping concentration; (iv) optimizing the field-stop (buffer) layer profile to reduce the peak electric field in the back side of the device; (V) using deeper p-type junctions at the surfaces to move the high electric field away from the electrodes; and (VI) thickening the gate trench oxide to alleviate the electric field strength at the trench bottom and the top side of the drift region. However, these approaches often lead to worse electrical performance trade-offs, e.g., poorer diode reverse recovery softness and higher on-state losses and hence worse Vce,sat (collector-emitter saturation voltage) and Eoff (turn-off loss).